The continent of Africa is sitting on a vast reservoir of groundwater in underground aquifers holding 100 times the amount found on the surface, scientists say.

Researchers from the British Geological Survey and University College London have mapped the amount and potential yield of groundwater resources for the entire continent.

While rivers and lakes are subject to seasonal changes that can limit their availability for people and for agriculture, the new maps suggest many countries currently considered to have limited supplies possess substantial groundwater reserves, the researchers said.

"Where there's greatest ground water storage is in northern Africa, in the large sedimentary basins, in Libya, Algeria and Chad," Helen Bonsor from the British Geological Survey told the BBC.

"The amount of storage in those basins is equivalent to 75 meters (250 feet) thickness of water across that area --- it's a huge amount."

Because of climate changes that turned the Sahara into a desert over centuries, many of the aquifers underneath it were last filled with water more than 5,000 years ago, the researchers said.

However, widespread drilling of large wells to tap these hidden resources might not be the best strategy, they said, citing concerns large-scale operations could rapidly deplete the aquifers.

Sometimes slower means of extraction can be more efficient, Bonsor said.

"Much lower storage aquifers are present across much of sub-Saharan Africa," she said.

"However, our work shows that with careful exploring and construction, there is sufficient groundwater under Africa to support low yielding water supplies for drinking and community irrigation."